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 Post subject: Heathkit WA-P2 power supply build questions
PostPosted: Aug Mon 09, 2021 12:41 am 
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Location: 20653
I am building a power supply to power a Heathkit WA-PS preamp using D-Labs schematic and have a few questions.
I wanted to build it out of stuff I have here so I didn't use the parts he used.
I used the transformer from a Arvin 751 that was parted out versus the one he used. I measured its no load voltage and the Hv is ~640V.
I couldn't find a 4H choke but I have a 11H.
I have 2 39UFD versus 33UFD.
Do any of the above changes have significant impact to the design? The WA-p2 wants ~320vdc

I want to set the voltage on the output but need to build a dummy load. I know R=E/I and I was going to use 320v for no better reason than that's what I want. I am not sure what to use as the current value to estimate the load resistor value?

Here is a close up of the Arvin schematic around the transformer and a link to the 751 schematic.
Attachment:
arvin 751.png
arvin 751.png [ 13.93 KiB | Viewed 1309 times ]


http://www.nostalgiaair.org/Resources/261/M0001261.htm

Here is snapshot of Terry's (DLabs) schematic and youtube video link
Attachment:
DLAB_PS.png
DLAB_PS.png [ 293.85 KiB | Viewed 1309 times ]


https://www.youtube.com/watch?v=aflnSukFdsY


Thanks
Andy


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Mon 09, 2021 4:48 pm 
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Quote:
I am not sure what to use as the current value to estimate the load resistor value?


What's the current rating of the Arvin transformer? Probably not more than 100 mA.

Rich


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Mon 09, 2021 5:28 pm 
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Try this, or one of several other sites that go into designing choke DC power supplies.

https://www.w8ji.com/choke_input_power_supply.htm

There's more to it than is readily apparent. There are some older ARF threads on this as well but I didn't look them all up.

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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Mon 09, 2021 5:59 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4712
Location: Lexington, KY USA
The WA-P2 is an audio preamp from the 1950s, intended to draw power from a separate power amplifier.
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Screenshot from 2021-08-09 12-50-00.png [ 294.06 KiB | Viewed 1255 times ]

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Heathkit-WA-P2-manual.pdf

( From a useful source for info: https://www.vintageshifi.com/repertoire-pdf/Heathkit-4.php)

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The Arvin Transformer is wildly larger than necessary, but will serve well to stabilize your system in a high wind.

With such a light B+ load, the power supply output will probably be much higher than 320V. This may be OK. If you want it lower, you might try removing the first filter capacitor to make a choke input filter. If the voltage is then too low, the capacitor needs to go back in. A series resistor right after the rectifier filament will reduce both the DC output voltage and the ripple voltage.

You might want to start by checking the 6.3V for the tube heaters. The light load on the transformer, combined with higher AC line voltage, may make this run high. Reduce the AC on the transformer primary to get this in line, then work on the DC output.

This is a case where a bleeder resistor across the B+ would be a good idea.

Ted


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Mon 09, 2021 8:46 pm 
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Joined: Jan Tue 29, 2019 7:01 pm
Posts: 483
Location: 20653
Barry H Bennett wrote:
Try this, or one of several other sites that go into designing choke DC power supplies.

https://www.w8ji.com/choke_input_power_supply.htm

There's more to it than is readily apparent. There are some older ARF threads on this as well but I didn't look them all up.



That link is incredible. It will take a long time to digest the insights/technical prowess represented there.

Quote:
The Arvin Transformer is wildly larger than necessary, but will serve well to stabilize your system in a high wind.

With such a light B+ load, the power supply output will probably be much higher than 320V. This may be OK. If you want it lower, you might try removing the first filter capacitor to make a choke input filter. If the voltage is then too low, the capacitor needs to go back in. A series resistor right after the rectifier filament will reduce both the DC output voltage and the ripple voltage.

You might want to start by checking the 6.3V for the tube heaters. The light load on the transformer, combined with higher AC line voltage, may make this run high. Reduce the AC on the transformer primary to get this in line, then work on the DC output.


I figured the Arvin was overkill. THanks for the advice/options.

Quote:
This is a case where a bleeder resistor across the B+ would be a good idea.


After looking at the link Harry provided. I think the load resistor is something in the 30k region. Does that seem reasonable?


Andy


Last edited by Govee on Aug Tue 10, 2021 2:56 pm, edited 1 time in total.

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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Mon 09, 2021 10:23 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4712
Location: Lexington, KY USA
30k will draw 10mA at 300V, so might serve as a substitute load instead of the actual preamp.

However, the preamp may not draw exactly 10mA, so the results won't be exact.

On the other hand, the preamp will work perfectly well over quite a range of B+ voltages.

Remember to take into account the power the resistor will be dissipating. 10mA and 300V is 3W. You want a resistor rated for quite a bit more, or it will get very hot. 10W would not be out of line.

A bleeder resistor could be as low as 30k, or perhaps as much as 100k.

Ted


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Tue 10, 2021 4:28 pm 
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This is not specifically about your power supply question but nevertheless an interesting read: It tells you how to convert the WA-P2 for driving lower input impedance solid state amplifiers. Good to add these infos to your WAP-2 documents/manual, it could be useful some day. (scan from Radio-Electronics, May 1967 issue, p.48)

Also, since this topic is not related to Test Equipment I think this thread should be moved either to "Vintage Audio...", "Homebrew..." or "Repair-restoration" for better reactions and visibility.


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WAP2.Conv.jpg [ 425.97 KiB | Viewed 1212 times ]
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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Wed 25, 2021 2:29 pm 
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Joined: Jan Tue 29, 2019 7:01 pm
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Location: 20653
I was wondering if I put this in the wrong category. I was thinking it just a DC power supply so maybe this is the right place but I cant argue it could go in one of the others.

I put together a 25k vice 30k load (parts on hand problem) and checked the voltage on the output and it was over 400VDC. I put a 10k resister in series with the load and voltage at the load dropped to about 310VDC. I was trying to build this out of parts on hand but I had to order the 10k resistor as only have a 50w (I used this in the test) and 1/2 watt on hand. I ordered a few 10k 2 watt and 3 watt resistors to have on and for this PS.

Andy


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Thu 26, 2021 5:10 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4712
Location: Lexington, KY USA
What is your 6.3V heater voltage measureing?

What B+ voltage to you get with a choke input filter?

Just disconnect the capacitor at the rectifier filament. You want to measure the loaded voltage; with no load, the voltage will still go very high.

A much larger choke inductance will further reduce the output voltage, but only try what you have on hand. A single-ended audio output transformer primary winding might work here.

The next thing to try is resistance between the rectifier and the input to the filter. Something like 2k ohms might be a starting place. This resistor will see several watts.

Ted


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Thu 26, 2021 6:19 pm 
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Joined: Jan Tue 29, 2019 7:01 pm
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Location: 20653
I didn't measure the 6.3v. I will go back and do that. I did have a 6v bulb on it.

The only b+ measurements I did was with the 25k load across pins 3 and 5. The first one I had over 400v. The second time I put a 10k dropper between the the second cap and pin 5. This gave me ~310v at the load resistor.

Quote:
Just disconnect the capacitor at the rectifier filament.

What cap is this? I don't have a rectifier filament. If it's not clear I built the ps with the diode rectifiers.

Andy


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Thu 26, 2021 7:27 pm 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
I don't have a rectifier filament. If it's not clear I built the ps with the diode rectifiers.

All of the schematics seen above use diode rectifiers. The 5Y3 is a dual diode. Since there is no separate cathode sleeve, the filament of the 5Y3 is the cathode of the tube's two diodes.

The "first" filter capacitor is the one nearest the rectifier, be the rectifier a tube or solid state. This capacitor is charged directly by the rectifier and will tend to charge up near the peak voltage of the rectified AC. If the choke is large enough, the capacitor after the choke will charge to more nearly the average of the voltage at the rectifier.

On another note, if you have a limited selection of resistors, series or parallel combinations may give you the value you need.

Ted


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Thu 26, 2021 7:40 pm 
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Joined: Jan Tue 29, 2019 7:01 pm
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Sorry Ted . I meant to say I built the one with silicon diodes vice the 5y3. So the reference to the filament was being lost on me as I wasn't bright enough to make the connection as you describe. I know see what you was saying.
I created the 25k using 3 75k. I didn't have 5 or 10 watt resistors at any value close to the 30k even when added in series. Most are below 1k. I had some 75k 50 watt resistors so I built a load resistor from them.

Thank you for the assistance.

Andy


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Fri 27, 2021 7:41 pm 
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Location: Thornhill, Ontario, Canada
Tubologic wrote:
... It tells you how to convert the WA-P2 for driving lower input impedance solid state amplifiers.... (scan from Radio-Electronics, May 1967 issue, p.48)...
Thanks for this... very good topic. I have added cathode-follower outputs to several preamps and FM tuners so as to feed s/s inputs, e.g. power amps, of around 47K.
Cheers,
Roger

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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Sat 28, 2021 7:59 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1561
I recently built a power supply with a similar arrangement, but a lower HT voltage. The transformers and chokes you require are available from AES, made by Hammond, and the chassis from them or Hammond.

You will have to select the correct transformer for the output voltage you require. Of note, the circuitry for this type of supply is very simple, it is much more of a task in mechanical engineering and wiring safety being a line powered supply, there are a few tips in the article which could help you get a good result:

https://www.worldphaco.com/uploads/POWE ... _1-10A.pdf

The exact DC voltage you get under load depends on the particular transformer and its voltage and winding resistances, the properties of the particular rectifiers you use, be they solid state, or directly or indirectly heated tube types, the capacitor values, especially the one directly on the rectifier output, the properties of the choke too, and since it is an unregulated supply, the line voltage in your locality. It can be modeled on paper, then trialed with practical experiment, or a circuit simulator gives a quick analysis.


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 Post subject: Re: Heathkit WA-P2 power supply build questions
PostPosted: Aug Sat 28, 2021 12:15 pm 
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I used to do what you need to adjust the power supply to the preamp, when I was building some preamp and tone control stages straight out of the schematics chapter of an RCA Receiving tube Manual, vintage of 1968. You can learn a great deal of useful information by ginning up those circuits. Of course there is more than ample info regarding power supply design, as well as physical layout and construction guidelines.
I will describe here what I did to adjust a basic DC supply to your preamp, but not how to wire the supply itself.
Also, the current requirements of that preamp will be so low that you won't be seeing any issues such as insufficient current due to a smaller transformer than is proper. Quite to the contrary.
The basic Pi circuit, C-R-C capacitor-input filter is a good design for you needs.
When you have the preamp wired in the configuration it will be used in, to make the desired B+ voltage to the preamp, you need to place a resistor between the output of the Pi filter and the preamp B+ input, which will reduce the voltage to what the preamp needs, and then connect another electrolytic cap to ground at the other end of this resistor, which will stabilize the voltage at that point. You'll have to experiment with the value of this resistor to derive the desired voltage. Do this with the cap after the resistor connected. I'm an enthusiast of overkill, so I would try a 10 uf here for starts. I don't think you will see any appreciable sag once the preamp is passing a signal. If so though, up the capacitance until the B+ at the preamp doesn't sag with load. I doubt you'll see that with a 10 or 20 uf connected.
If you would a copy of the RCA receiving tube manual I have it as a digital file and can put it on google drive or such.
Regards, C-N-E

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